Disopyramide, an antiarrhythmic drug, has been reported to cause hypoglycemia; however, its mechanism of action remains unclear. Pre-existing factors that increase the concentration of the drug in the blood increase the risk of hypoglycemia. Furthermore, other factors can also increase the risk of hypoglycemia even when disopyramide levels are in the therapeutic range. It has been proposed that disopyramide-induced hypoglycemia is caused by stimulation of insulin secretion due to the inhibition of the pancreatic beta-cell ATP-sensitive K+ channels. We report a case of severe disopyramide-induced hypoglycemia in a nondiabetic 72-year-old woman on hemodialysis. Concentrations of counter-regulatory hormones, serum insulin, and C-peptide were measured. From these data, it appears that disopyramide-induced hypoglycemia results from sustained endogenous insulin secretion, with a concomitant inadequate counter-regulatory response. Although hypoglycemia occurs infrequently in patients treated with disopyramide, this adverse effect is clinically important and potentially life-threatening. Evidence suggests that disopyramide-induced hypoglycemia results from endogenous insulin secretion and can occur in patients with therapeutic blood concentrations of the drug. Patients at risk include those with renal impairment, advanced age, and malnutrition, and blood glucose levels should be monitored carefully in such patients.